1. What is the link that shows up and what should be the actual link?
2. Create a new style with no parent:
- Styles & Templates > Style Manager > Add New Style
- Parent Style: No Parent Style
- Title: Default vBulletin
- Allow User Selection: Yes
- Save
Then sign in using this totally default vbulletin style - do you still have the same problem?
3. What do you have set for AdminCP > Settings > Options > Cookies > Cookie Domain and do you allow users to login via both
www.site.com and just site.com?